All cellular communications systems are divided into cells, where User Equipments (UEs) are served by one, or when in soft(er) handover, several base stations. Each base station may serve UEs in more than one cell. The important point from a positioning and navigation perspective is that the cell where a specific UE is located is known in the cellular system. Hence, after determination of the geographical area covered by a specific cell, it can be stated that the UE is located somewhere within said geographical area, as long as it is connected and the reported cell identity of the serving cell is equal to the cell identity corresponding to the particular geographical area.
An example of positioning within a Wideband Code Division Multiple Access (WCDMA) cellular system operates briefly as follows, assuming that the positioning operates over the Radio Access Network Application Part (RANAP) interface. The procedures are however similar for e.g. the Global System for Mobile communications (GSM) and Code Division Multiple Access 2000 (CDMA 2000).
A message requesting a location estimate is received in the Serving Radio Network Controller (SRNC) over the RANAP interface. The quality of service parameters of the message is assumed to be such that the Radio Network Controller (RNC) selects the cell identity positioning method. The SRNC determines the serving cell identity of the UE to be positioned and retrieves a pre-stored polygon that represents the extension of the serving cell. The SRNC sends the resulting cell polygon back to the core network over the RANAP interface, using a cell polygon format in a location report message.
It should, however, be noted that due to the complexity of the radio propagation, the cell polygon format is only an approximation of the extension of the true cell. The selection of the polygon format is dictated by the need to have a reasonably flexible geographical representation format, taking, e.g., computation complexities and reporting bandwidths into account.
Since the polygon format approximates the cell extension, the polygon is normally pre-determined in a cell-planning tool to represent the cell extension with a certain confidence. The confidence is intended to represent the probability that the UE is located within the polygon, conditioned on the fact that it is connected to the cell that is represented by the cell polygon. The underlying off-line calculation of the cell polygon can e.g. be based on coverage simulations of varying levels of sophistication. However, the end result is normally not very reliable when the confidence of the calculated cell extension is considered.
The accuracy of the cell identity positioning method is mainly limited by the size of the cell, something that prevents it from being used in more sophisticated navigation applications. Its main advantages include a very low response time as well as the fact that it is widely spread and always available where there is cellular coverage. The cell identity method is also straightforward to implement and has no UE impact. The advantages has lead to an interest for the development of Enhanced cell identity (E-cell ID) positioning methods that aim at enhancing the accuracy of the basic cell identity method at the same time as the advantages of the method are retained.
One principle for E-cell ID positioning aims at combining the cell extension model with a distance measure. One possibility towards this end is Round Trip Time (RTT) measurements. In the RTT measurement principle, the travel time of radio waves from the Radio Base Station (RBS) to the UE and back is measured. The RTT method alone defines a circle around the RBS. By combining this information with the cell polygon, left and right angles of the circle can be computed.
Another idea for enhanced cell identity positioning has been to use pre-calculated maps of the regions where the UE is in soft(er) handover with one or several cells. Such areas are significantly smaller than the whole cell opening up for a better accuracy of the determined position. Normally these maps are pre-calculated in the planning tool, exactly as the ordinary cell polygons.
Measurements of the path loss using measurements of the signal strength in the UE with respect to specific radio base stations (RBS) constitute another alternative. However, these measurements suffer from shadow fading effects of several dBs, which reduces accuracy when used for positioning purposes. Assuming an 8 dB fading effect the accuracy is limited to about half the distance corresponding to the path loss measurement.
In some situations high-precision positioning is required. In the present disclosure, “high-precision positioning methods” are defined to denote positioning methods that have a potential to meet the North-American E-911 emergency positioning requirements. Methods that meet these requirements are capable of obtaining positioning accuracies of:                either (terminal based) 50 meters (67%) and 150 m (95%),        or (network based) 100 meters (67%) and 300 m (95%).        
Assisted Global Positioning System (A-GPS) positioning is an enhancement of the Global Positioning System (GPS). GPS reference receivers attached to e.g. a cellular communication system collect assistance data that, when transmitted to GPS receivers in terminals connected to the cellular communication system, enhance the performance of the GPS terminal receivers. Typically, A-GPS accuracy can become as good as 10 meters. Additional assistance data is collected from the cellular communication system directly, typically to obtain a rough initial estimate of the position of the terminal together with a corresponding uncertainty of the initial estimate. This position is often given by a cell identity positioning step.
The Uplink Time Difference Of Arrival (UTDOA) positioning method is based on time of arrival measurements performed in several RBSs of transmissions from the UEs. The signal strengths are higher than in A-GPS, something that enhances the ability to perform positioning indoors. The accuracy of UTDOA is expected to be somewhat worse than that of A-GPS though, mainly because the radio propagation conditions are worse along the surface of the earth than when GPS radio signals are received from satellites at high elevation angles.